The purpose of this study is three-fold: first, to isolate, purify and chemically characterize the surface antigens of pathogenic Staphylococcis aureus isolated from human infections. Second, to study the immune response to these surface antigens in rabbits and mice. Special emphasis will be directed towards the identification of antibodies with opsonic properties. Attempts will also be made to determine the prevalence staphylococcal surface antigen antibodies in human serum. In these studies, attempts will be made to isolate heat-stable and heat-labile opsonins from human serum by affinity chromatography. These opsonins will be characterized by immunochemical techniques which includes immunofluorescense and ferritin-labelled electron microscopy. Third, to determine the feasibility of employing these surface antigens as a means of classifying S. aureus into distinct groups by the precipitin test. Further, attempts will be made to determine the possible effectiveness of purified anti-phagocytic antigens as immunogens in animals.